The present invention relates to an air separation method for supplying gaseous oxygen in accordance with the requirements of a variable demand pattern.
A variety of industrial processes have time varying oxygen requirements. For example, steel mini-mills utilize oxygen in the reprocessing of scrap steel. Since the scrap steel is processed by such mills in batches or heats, the demand for oxygen varies between a high demand phase during batch processing and a low demand phase between batch processing. In order to meet such oxygen demand requirements, the prior art has provided air separation plants that are designed to supply gaseous oxygen in accordance with a variable demand pattern having high and low demand phases. Such air separation plants can generally be said to store liquid oxygen during the low demand phase and to store liquid nitrogen during the high demand phase. Moreover, the liquid nitrogen and the gaseous oxygen product are produced by vaporizing the stored liquid oxygen against condensing gaseous nitrogen produced by the plant.
In one type of plant design, the gaseous oxygen product is directly supplied from the low pressure column of an air separation unit having a high pressure column operatively associated with the low pressure column by a condenser/reboiler. In such a plant design, the gaseous oxygen product is produced by evaporation of liquid oxygen in the low pressure column against condensation of gaseous nitrogen in the high pressure column. In another type of plant design condensation of nitrogen and evaporation of oxygen occur in heat exchangers external to an air separation plant rather than in low and high pressure columns of such a plant.
An example of the type of air separation plant in which the gaseous product oxygen is supplied from the low pressure column is described in "Linde Reports on Science and Technology", No. 37, 1984. The plant disclosed in this publication supplies gaseous oxygen at a nominal production rate by extracting vaporized oxygen from the low pressure column. The oxygen vaporizes against the condensation of nitrogen produced at the top of the high pressure column. A stream of the high pressure nitrogen is extracted from the high pressure column and is subsequently heated, compressed, partially cooled, and turboexpanded to supply plant refrigeration.
In the plant described above, the amount of high pressure nitrogen extracted to supply plant refrigeration is controlled to adjust the amount of gaseous oxygen supplied, either above or below the nominal rate. During the high demand phase, the amount of high pressure nitrogen extracted from the high pressure column is reduced below that which is required to be extracted to produce gaseous oxygen at the nominal production rate. As a result, there is an increase in the degree to which liquid oxygen in the bottom of the low pressure column evaporates and high pressure nitrogen at the top of the high pressure column condenses. This produces an increase in the amount of liquid nitrogen collected at the top of the high pressure column which is extracted and stored in a storage tank. Liquid oxygen, stored in another storage tank during the low demand phase, is supplied to the low pressure column to replenish oxygen in the bottom of the low pressure column. During the low demand phase, the amount of high pressure nitrogen extracted from the high pressure column is increased over that required to be extracted in the production of oxygen at the nominal rate. This increases the amount of liquid oxygen collected at the bottom of the low pressure column because there is less high pressure nitrogen at the top of the high pressure column to condense. The increased amount of liquid oxygen collected in the low pressure column is extracted and stored for use in the high demand phase while previously stored high pressure nitrogen is introduced to the top of the low pressure column as reflux to wash down the oxygen and to add refrigeration. Processes of this design are limited by a ratio of maximum oxygen production to average oxygen production of about 1.5, owing to the means effected for varying the oxygen production rate.
An example of an air separation plant in which evaporation and condensation of oxygen and nitrogen takes place in added heat exchangers and vaporizers is described in U.S. 3,273,349. The air separation plant described in this patent is designed to supply liquid oxygen and waste nitrogen at nominal rates of production. During periods of low or no oxygen demand, liquid oxygen is stored in a storage vessel while liquid nitrogen, previously produced and stored during the high demand period is returned to the air separation plant for use as reflux to the low pressure column thereof. During periods of high demand, liquid oxygen from the storage vessel is pumped through a heat exchanger while waste nitrogen is compressed and is countercurrently passed through the heat exchanger. As a result, the liquid oxygen is vaporized for supply as product and the compressed nitrogen condenses and is stored for use during the low demand period.
Design and operational problems exist in variable demand oxygen plants in which gaseous oxygen is supplied directly from the low pressure column. For instance, optimization of the hydraulic design of the column and oxygen recovery over the full extent of the demand pattern are highly problematical. A major operational problem is that it is difficult to control the purity of the oxygen being recovered. Also, the oxygen that is recovered is supplied at too low a pressure to be practically utilized in an industrial process. As a consequence, the pressure of the oxygen must be increased by use of an oxygen compressor. It is to be noted that in variable demand oxygen plants in which oxygen is supplied by pumping liquid oxygen through a heat exchanger or vaporizer, the oxygen is supplied at a usable working pressure without the use of an oxygen compressor. However, while equipment costs may at least in part be saved in such a plant design, operating costs are increased in that there are energy losses involved in vaporizing oxygen and condensing nitrogen outside of the cold box. As may be appreciated, both type of Plant designs involve the use of additional compressors, heat exchangers and etc. that in any event significantly add to plant cost and complexity.
As will be discussed the present invention provides a method that is capable of supplying gaseous oxygen over a variable demand pattern at usable working pressures and over a wider range of demand than that contemplated in the prior art. While being totally integrated, the method of the present invention is far less complex than that involved in variable demand oxygen plants of the prior art. Additionally, column operation in a process of the present invention is very stable. This eliminates the design and operational problems associated with variable oxygen demand plants in which the oxygen is supplied directly from the low pressure column.